About the MAC table on switches?

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http://serverfault.com – I'm learning the network basics at my university this semester. I had a question on the mid-term exam and unfortunately ,I answered it wrong. I have a final exam about an hour from now, and I still don't know what the correct answer of that question is. However, here is my question: A network of several PCs, switches, and one router. PC 1 has a MAC address X. PC 1 usually sends packets to PC 5 and these packets pass through Switch Z. Inspecting the MAC address table of Switch Z for a very long period revealed that the PC 5 MAC address never shown in the table. Is there anything wro (HowTos)